This invention relates to a merged-transistors switch and more particularly to an integrated circuit (IC) driver for an inductive load such as a PM motor load.
A permanent-magnet type stepping motor, otherwise known as a PM motor, has a rotor that includes a permanent magnet. Magnetic flux reversal occurs in each stator pole, and therefore each pole must be provided with two separate but closely coupled windings, one for each direction of magnetization. It is common practice to employ a transistor switch to control each winding, driving DC current first through one of the windings and then through the other so that each winding is excited when the other is not.
In FIG. 1 there is shown an elementary circuit for driving a PM stepping motor having two poles 12 and 14. Two bifiler windings 16 and 18 are connected together at opposite-polarity ends thereof. The other winding ends are connected to the emitters of two NPN transistors 20 and 22, respectively at the other pole 14 and the two windings 24 and 26 are similarly related to transistors 28 and 30.
The appropriate driving sequence for this circuit is shown in the Table of FIG. 2 wherein, S.sub.1, S.sub.2, S.sub.3, S.sub.4 represent successive phases or periods of time and wherein a "1" denotes the presence of a drive current in a winding and a "0" represents zero drive current.
The transistors 20 and 22 at pole 12, for example are thus turned on alternately to drive the PM motor. During a transition, e.g. from phase 3 to phase 4, when one transistor, e.g. 22, is turning off and the current through the transistor (22) essentially ceases flowing while, due to tight mutual coupling between the winding 16 and 18, a back-e.m.f. in the other winding 18, begins to establish an equal value current conducting through diode 34. Diode 34 conducts momentarily in the forward direction and shunts the resulting transient current away from its companion transistor 20.
At this transition, a large negative voltage spike appears at the emitter of (the off) transistor 22. It is conventional to provide additional diodes, e.g. 35, to limit this negative spike violtage. A battery 36 is connected between ground and the collectors of transistors 20,22,28 and 30.
This invention concerns a merged switch that is particularly suitable for switching currents in a load having a high L/R ratio. For example, the switch of this invention can provide the same function as do the pair, transistor 20 and shunting diode 34 (or the pair 22 and 32) in FIG. 1.
It is an object of this invention to provide a more efficient merged-switching transistor device for use in driving a load having a high L/R ratio, such as a PM motor.